Insulated gate bipolar transistors (IGBTs) are commonly used in the inverter circuit section of motor drive circuits. Inverter circuits of this kind are shown, for example, in U.S. Pat. No. 5,825,641.
When the motor of a motor drive circuit turns very slowly, the power output of the inverter is at very low frequency, for example, 3 Hz. The relationship between IGBT transient thermal impedance and its peak junction temperature rise is a function of the output frequency of the motor drive inverter. The same output current is generally required at all output frequencies. The rated output current of an IGBT die for a particular circuit is, therefore, limited to that obtainable at the lowest output motor current frequency, typically 3 Hz, since the junction temperature is at its highest at this frequency. More specifically, while the IGBT device will switch at a frequency between 1 KHz to 20 KHz, the meter current which it carries may have a frequency as low as 3 Hz at slow spaced rotation.
If the transient thermal impedance can be reduced, the ratio of peak to average junction temperature will be reduced, and increased output current can be obtained at 3 Hz for a given IGBT die.
More specifically, the ratio between peak and average junction temperature at 3 Hz can be approximated as: ##EQU1##
where R.sub.th75ms is the transient thermal resistance for a 75 ms current pulse and R.sub.thDC is the DC thermal resistance of the IGBT. T.sub.JPK is the peak junction temperature and T.sub.JAV is the average junction temperature. For IGBT devices presently made by the International Rectifier Corporation of El Segundo, Calif., R.sub.th75ms /R.sub.thDC is about 0.8, which gives T.sub.JPK /T.sub.JAV =2.8. It would be desirable to reduce the transient thermal resistance since that would allow the use of a smaller die for a given motor controller application.